Suppress cppcheck unusedFunction warnings for CMHG entry points

Just about every C module in the source tree uses CMHG to interface with the
kernel. However, cppcheck doesn't understand CMHG files, and so it will
incorrectly identify any C entry points from the CMHG object file as unused
functions.

The main command-line suppression options for cppcheck are:
* --suppress=unusedFunction : blanket suppression of the warning for all
  files. This is undesirable because it would mean we miss many examples of
  dead code, and ironically, since modules commonly feature in ROMs which
  are space-constrained, it's particularly valuable to identify dead code
  in these cases.
* --suppress=unusedFunction:[filename] : better, but a pain to implement the
  CI job for (we'd need to pass it the relevant filename(s) somehow), a pain
  to maintain (for every module, we'd need to identify the relevant file(s))
  and could still miss some dead code.
* --suppress=unusedFunction:[filename]:[line] : solves the dead code problem
  but at the cost of being even more hassle to maintain, due to having to
  keep line numbers up-to-date.

Compared to these options, inline suppression markers look very attractive.
However, objections have been raised to these also, so here we use a new
feature of cppcheck 1.84 (available now that we have upgraded the GitLab
runner machine to Ubuntu 20.04): the more verbose but more flexible option
of passing a suppression specification to cppcheck in XML format. The XML
file itself is generated during the "make standalone" command that is
performed as part of the CI job. For non-module components, the XML file is
not generated, and the option to cppcheck is silently removed.

Required RiscOS/BuildSys!44
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